Process for the preparation of crystalline sodium silicates

Abstract

A process for the preparation of crystalline sodium silicates with a molar ratio SiO 2 /Na 2 O of 1.9:1 to 3.5:1 from amorphous sodium silicate is described. To water-containing amorphous sodium silicate with a molar ratio SiO 2 /Na 2 O of 1.7:1 to 3.75:1 and a water content of 5 to 95% by weight is added 0.01 to 30 parts by weight of the crystalline sodium silicate to be prepared (per 100 parts by weight of Na 2 O+SiO 2 in the water-containing amorphous sodium silicate). The reaction mixture is dehydrated by heating. The dehydrated reaction mixture is kept at a temperature of at least 450° C., but below the melting point, until the sodium silicate has crystallized.

I claim: 
     
       1. A process for the preparation of crystalline sodium silicates with a molar ratio SiO 2  /Na 2  O of 1.9:1 to 3.5:1 from amorphous sodium silicate at elevated temperature, which comprises adding to water-containing amorphous sodium silicate with a molar ratio SiO 2  /Na 2  O of 1.7:1 to 3.75:1, and a water content of 5 to 95% by weight, 0.01 to 30 parts by weight of the crystalline sodium silicate to be prepared (per 100 parts by weight of Na 2  O+SiO 2  in the water-containing amorphous silicate), dehydrating the reaction mixture by heating, and keeping the dehydrated reaction mixture at a temperature of at least 450° C., but below the melting point, until the sodium silicate has crystallized. 
     
     
       2. The process as claimed in claim 1, wherein 1 to 15 parts by weight of the crystalline sodium silicate to be prepared (per 100 parts by weight of Na 2  O+SiO 2  in the water-containing amorphous sodium silicate) are added to the water-containing amorphous sodium silicate used. 
     
     
       3. The process as claimed in claim 1, wherein the amorphous sodium silicate used has a water content of 15-26% by weight. 
     
     
       4. The process as claimed in claim 1, wherein the sodium silicate used has a water content of 50-70% by weight. 
     
     
       5. The process as claimed in claim 1, wherein both the amorphous water-containing sodium silicate and the added crystalline sodium silicate have a molar ratio SiO 2  /Na 2  O of 1.7:1 to 2.3:1. 
     
     
       6. The process as claimed in claim 1, wherein the dehydrated reaction mixture is comminuted mechanically and is then kept at a temperature which is at least 450° C. 
     
     
       7. The process as claimed in claim 1, wherein the added crystalline sodium silicate has a molar ratio SiO 2  /Na 2  O of about 2:1. 
     
     
       8. The process as claimed in claim 7, wherein the added sodium silicate has X-ray spectra in accordance with Table 2. 
     
     
       9. The process as claimed in claim 8, wherein the crystallization is carried out in the range from 700° to 800° C. 
     
     
       10. The process as claimed in claim 9, wherein the dehydrated reaction mixture is heated at 700°-770° C. for 1-80 minutes. 
     
     
       11. The process as claimed in claim 9, wherein the dehydrated reaction mixture is heated at 770°-800° C. for 1-12 minutes.